In the leukemogenically irradiated mouse, the onset of leukemic disease is preceded by a nonmalignant phase, or preleukemic state, following recovery from the acute radiation syndrome. Previous work suggests that removal of feedback inhibition, exerted by mature hemic cells over their immature precursors, accounts in part for the proneness of the irradiated mice to develop leukemias. The objective of this project is to manipulate this preleukemic change experimentally, with the purpose to postpone or to prevent radiogenic leukemias.